A folding method on the one hand and a folding machine on the other hand have become known through U.S. Pat. No. 5,611,133 A as well as U.S. Pat. No. 5,752,304 A. An exterior and an interior preformed metal panel are positioned between lower and upper die halves, wherein the die halves are subsequently jointed and the preformed panels are restrained within the die. A pre-folding of a corner area of the exterior metal panel is performed subsequently. This process is followed by a finish folding process of the pre-folded area. Provided in a non-folded area is at least one welding facility that joins both metal sheets by spot welding.
WO 99/03634 discloses a method and a device for welded joining of metal parts overlapping each other. Proposed therein is a resistance welding process for welding connection of metal sheets, with one end area of a metal sheet being folded around the other metal sheet. Accommodated in the area of the folding machine is at least one welding head comprised of an electrode with welding tip. In the folded area, one of the metal sheets has a kind of a projection. The connection is now achieved in that the welding head exerts a force on the fold so that the fold is fixed between the electrode and a backing plate. Subsequently, a pulsating welding current is generated that joins the two metal sheets with each other.
A method and a device for establishing a projection welding connection between metal sheet parts has become known through EP 1 038 627 BI. A projection having a triangular cross-section is preformed in one of the metal sheets to be welded. The metal sheet plates are laid one above the other so as to overlap each other. A fold is generated in the area of the projection. The metal sheet plates are welded together at the point of the projection by applying an electric current pulse of short duration, with the electric current being supplied from the side of the pressure unit to the metal sheet plate lying on top and being discharged from the metal sheet plate lying at bottom.
DE 199 27 208 B4 discloses a method and a device for connecting metal sheets at the edges, more particularly of car body metal sheets in automobile manufacturing, with the edge of an exterior metal sheet plate being folded beyond the edge of an interior metal sheet plate, and wherein the edges are pressed together and wherein the edge of at least one of the metal sheet plates is provided with embossed projections prior to folding.
While pressing the metal sheet plates together, the projections are connected by projection welding to the relevant other plate, utilizing an electrode arranged at the folding jaw. This actually compact structural form bears a disadvantage in that handling of metal sheet plates within the device is complicated.